1. Field of the Invention
The present invention concerns a method for operation of a magnetic resonance (MR) system and in particular concerns a method for generation of MR images using gradient echoes.
2. Description of the Prior Art
For specific diagnostic questions, a T2* weighting is desired in magnetic resonance tomography. T2* stands for the time constant for the loss of the phase coherency of the spin due to an interaction of magnetic field inhomogeneities making use of this phase coherency loss, with and spin-spin transverse relaxation. Gradient echo sequences are typically used for long echo times TE being required for a T2* weighting. This in turn means that long repetition times TR and small excitation angles α (to avoid saturation effects) must be used.
Furthermore, a pulse sequence is known as a MEDIC sequence is known described, for example, in DE 198 08 662. In such a MEDIC sequence, a number of bipolar readout gradients are switched in series, and each bipolar readout gradient generates a gradient echo. Given a number of bipolar gradients, this leads to gradient echoes at different echo points in time TE.
Furthermore, use of the chemical shift in NMR spectroscopy is known. The chemical shift is the shift in the resonance frequency of different tissue dependent on the type of the chemical bond in which the atom under investigation participates, the resonant frequency shift being proportional to the field strength. In the human body, only hydrogen nuclei of the free water and in fat compounds contribute to the signal, and thus to the MR image. The relative resonant frequency difference δ is approximately 3 ppm (parts per million), meaning 130 Hz given a field strength of 1.0 T.
In the examination of tissue with fat and water portions, the tissue portion of the water and the tissue portion of the fat have different resonant frequencies. These different resonant frequencies mean that, upon deflection of the spins by a radio-frequency excitation pulse, these spins precess with different frequencies. This leads to the situation that the phase position of the water signal portions varies with the time relative to the phase position of the signal portions of the fat. Phase coherency of the signal portions of the water and of the fat predominate dependent on the time, which means that the signal portions of both tissue parts appear in the same direction (what are known as in-phase conditions). Furthermore, at another point in time it can occur that the phase positions of the two tissue types are offset by 180° relative to one another, such that the magnetization of the water tissue is situated opposite (counter) to the magnetization of the fatty tissue (what are known as opposed-phase conditions).
If the phase position of both tissue types is the same, the resulting overall magnetization (and thus the overall signal) is greater while the overall signal is reduced given opposite alignment of the two magnetizations. This leads to different contrasts in the magnetic resonance image, dependent on the echo time.
As mentioned above, in the MEDIC sequence a number of gradient echoes are generated at different echo times TE. After Fourier transformation and absolute value generation (magnitude generation), the normalized sum of the squares of the echo data is output as an image. The phase position of the two tissue types (fat and water) relative to one another changes continuously with time, such that given multi-gradient echo sequences at different times T various image contrasts arise. Conventionally, the echoes have been measured in succession in the MEDIC sequence without the arising phase positions of the fat protons and water protons being taken into account. The composite MEDIC image generally has a mixed contrast.